According to reports, if you pre-ordered MGS4 from GameStop then you can look forward to receiving a special bonus disc that includes a code for the MGO BETA and access to some special features.
To quote: Also posted on the PS3 general board but we got our Metal Gear Saga Vol 2. in today at my work. Inside you get a card that says he beta will be available to download April 17 and gameplay will be available from April 21st - May 5.
You just enter the voucher code included. The dvd covers the series and the main characters of the story, it also includes all of the trailers that had been previously released.
The dvd goes as follows:
Chapter 001 - Prologue
Chapter 002 - Metal Gear 20th Anniversary
Chapter 003 - Big Boss - The origin of Snake
Chapter 004 - Liquid - The Second Snake
Chapter 005 - Solidus - The Third Snake
Chapter 006 - Solid - The First Snake
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